9v mains with battery backup the 790mbx heat alarm is mains powered with 9v alkaline battery back up and a push fit docking base system designed to ensure quick and easy installation of base unit to the mains feed supply at installation stage 1st fix and for quick and easy connection of alarm to base unit 2nd fix the push fit base provides spacious room for wiring in premises such as flats where concrete ceilings can be problematic top and side entries accommodate for either cable or mini